This invention relates generally to actuators for mechanical indicators of change of state in a hydraulically or pneumatically isolated system, and more particularly to a magnetic actuator for an indicator detecting a change of state, such as the passage of a pig, in pipelines, pressure containers and the like.
In industrial applications involving chambers or vessels that are isolated hydraulically or pneumatically from the operator's environment, it is often necessary to indicate a change of state in the pressurized or evacuated environment. As an example, in the pipeline industry, typically spherical objects known as "pigs" are introduced into the pipeline to indicate a change of transported fluid, or the end of a transmission or some other change of state. Mechanical detectors are placed in communication with the pipeline so as to be triggered when a pig passes by. Typically, the mechanical detector is in fluid communication with the pipeline itself. Thus, it is necessary to transmit the evidence of the passage of a pig from the internal pressurized environment, outside to the environment of the operator. U.S. Pat. No. 4,491,018, the disclosure of which is incorporated herein by reference, describes generally the use of pig detectors in the pipeline industry.
Several means are used to transmit the evidence of the internal change of state to the outside operator. As one example, the internal mechanical indicator may be attached to a translating shaft which translates, which shaft pierces the container of the pig detector and translates on the outside of the pig detector in some noticeable or measureable way. An obvious drawback of this method is that in piercing the container of the pig detector, the shaft introduces leaks into the system. These leaks must be countered by seals, which often fail, or need to be replaced. Another method, also described in the above referenced patent, involves incorporating a permanent magnet on the translating internal shaft, and detecting a change of magnetic field with a magnetic reed switch attached to the outside of the detector housing in a zone where the magnetic flux will change. A drawback of this method is that an electrical power source must be provided. If the detector is to be used away from permanent power sources, battery sources must be used, which must be continually checked and replaced. Further, the electrical method must be designed to eliminate shorting problems. Finally, this method is prone to failure due to corrosion, which is common in the environments where such detectors are used. Electric devices are also often complicated and expensive to maintain. Finally, if the desired indicator is to be observable visually at great distances, an additional power source must be used to provide mechanical work energy, or the power source powering the electrical device must be suitably sized to deliver the mechanical work energy.
Thus, the several objects of this invention include providing an actuator: that may be used with pressurized or evacuated closed containers without the need for seals or translation of members through a pressure barrier; that does not require provision of an external electric power source; that is resistant to failure due to corrosion; that is resistant to failure due to electrical short circuits; and that is relatively inexpensive to manufacture and maintain. Other objects of the invention will be apparent or will be explained in the following discussion.